Outcome measures such as mortality, cost and efficacy of treatment, and length of stay in the ICU have little significance if severity of hospital illness is not accounted for. Identifying high-risk patients has promoted the concept of scoring systems. Assessment of patient prognosis depends on the ability to convene an inception cohort of subjects at a uniform point in the course of their illness. Understanding the continuum of illness, including organ dysfunction and comorbid factors that are not accounted for in general severity scores, may enhance the potential accuracy and usefulness of models.